Agricultural equipment, such as four wheel drive loader, a tractor or a self-propelled combine-harvester, includes a prime mover which generates power to perform work, a shaft power conversion machine, and reversible energy storage. In the case of a four wheel drive loader, for instance, the prime mover is often a diesel engine that generates power from a supply of diesel fuel. The diesel engine drives a transmission which moves a ground engaging traction device, such as wheels or treads, to propel the loader, in some situation, across unimproved ground for use in construction. Such loaders include a hydraulic machine having a hydraulic pump which can be used for instance, to raise or lower a piece of equipment such as a bucket.
In the case of a tractor, the prime mover is often a diesel engine that drives a transmission which moves wheels or treads to propel the tractor across a field. In addition to providing power to wheels through a transmission, tractors often include a power takeoff (PTO) which includes a shaft coupled to the transmission and driven by the engine. The power takeoff can be used to drive a variety of work implements or accessories including pumps, plows, and harvesters.
Other work vehicles having prime movers include construction vehicles, forestry vehicles, lawn maintenance vehicles, as well as on-road vehicles such as those used to plow snow, spread salt, or vehicles with towing capability. While each of the work vehicles, including the work equipment described above, often include combustion engines as the prime mover, some vehicles have what is known as a hybrid power system. A hybrid power system integrates one or more reversible energy storage device(s) and one or more machines to convert power between the energy storage device(s) and one or more rotating shafts.
Generally, the hybrid power system delivers power and absorbs power using a reversible shaft power conversion machine. Examples of reversible shaft power conversion machines are electric motor/generator and hydraulic motors/pumps. The reversible shaft power conversion machine is configured to receive power from the hybrid energy storage device(s) and provide shaft power to assist a powertrain, and to receive power from the powertrain, such as from regenerative braking, for delivery into the hybrid energy storage device(s) When using a hybrid power system, it is still necessary to provide speed control or overall torque control, while simultaneously keeping the energy storage device within a desired stored energy level. While separate control governors can be used, one for the diesel engine which limits engine speed, and one for the reversible shaft power conversion machine, limiting motor speed, separate control governors can lead to difficulty with smooth torque control when one of the engine and hybrid power system reaches a maximum limit. In addition, optimization of the overall system is difficult. What is needed, therefore, is a control system having combined control for both the engine and the reversible shaft power conversion machine.